This chapter provides data on properties of quantum wells (QWs) for structures with HgTe, (Hg,Cd)Te, and (Hg,Zn)Te wells. The properties include band-gap energy, exciton binding energy and lattice constant. The chapter also gives information on electronic states, and electron-hole and excitonic transitions in QWs in a low-density regime. Modification of optical properties by internal and external fields is given. Infrared absorption spectra and photoluminescence spectra of multiple QWs (MQWs) at different temperature is figured out. Finally, it outlines high-density regime and nonlinear optics including second-order susceptibility, stimulated emission and lasing of quantum wells.